Temperature sensors

ABSTRACT

An ultrasonic distributed temperature sensor comprises an elongate ultrasonic waveguide in the form of steel or nickel wire 0.16 cm in diameter and up to 10 meters long, which is strung around the area whose temperature is to be monitored, eg an aircraft engine. The wire is provided with discontinuities in the form of annuli welded on to it to form annular flanges, which serve to partially reflect ultrasonic pules launched into one end of the wire. These flanges, known as &#34;posi-notches&#34;, divide the waveguide, and thus the area to be monitored, into a number of zones, the size of each zone being determined by the spacing of the adjacent flanges defining it. In operation, the temporal spacing of each pair of successive partially reflected pulses is a measure of the average temperature of the zone defined by the flanges producing that pair of pulses. In order to mount the waveguide in the area to be monitored, it is held by locating devices which engage the radial surfaces of some of the flanges, so locating the waveguide longitudinally, and by annular clamping devices which lightly grip the wire without introducing undesired stresses. In one embodiment, the locating devices and the mounting devices are basically very similar, and comprise an annular member having a bore containing at least three axially extending resilient strips which make line contact with the waveguide to support it.

This invention relates to temperature sensors, and is more particularlybut not exclusively concerned with ultrasonic distributed temperaturesensors for use in aircraft.

There are several areas in modern aircraft where it is desirable tomeasure the temperature distribution over the area, for example withinan engine housing or within an avionics bay, so that overheating in partof the area can be detected early enough for appropriate remedial actionto be taken. In this way, fires and other potential catastrophicfailures can be avoided.

One known form of distributed temperature sensor is described in U.S.Pat. No. 3 636 754, and comprises an elongate ultrasonic waveguide whichis intended to be strung out around the area to be monitored. Thewaveguide is provided with a number of longitudinally distributedreflecting means, e.g., notches, which divide it into zones definingcorresponding zones of the area whose temperature is to be monitored.Ultrasonic pulses launched into one end of the waveguide are partiallyreflected at each reflecting means to form echo pulses, and therespective time intervals between the receipt of successive echo pulsesresulting from a given launched pulse are measured in a counter/timer.Since the propagation speed of the ultrasonic pulses in the waveguide isa function of the temperature of the waveguide, the time intervalbetween two successive echo pulses resulting from a given launched pulseis a measure of the average temperature of the zone defined by the twosuccessive reflecting means which gave rise to those echo pulses.

Although this known form of distributed temperature sensor works well insome environments, it is difficult to use in an aircraft environment.Thus use of the sensor in an aircraft environment requires that thesensor be mounted so as to be able to withstand fairly harsh vibrationand shock-loading conditions, without moving longitudinally (since thatwould change the correspondence between the zones of the sensor and thezones of the area being monitored). However, radially gripping thewaveguide tightly to prevent longitudinal movement produces local radialstresses in the waveguide, and these stresses can produce spurious echopulses, i.e., echo pulses which are difficult to distinguish from thetrue echo pulses from the reflecting means.

It is an object of the present invention to alleviate this difficulty.

According to one aspect of the present invention, there is provided atemperature sensor comprising an elongate ultrasonic waveguide havingdistributed along its length a plurality of means for partiallyreflecting ultrasonic pulses launched into one end of the waveguide, andmeans for mounting the waveguide such that it extends through an areawhose temperature is to be monitored, wherein at least some of thereflecting means comprise outwardly projecting portions of the waveguideeach having a pair of opposed surfaces extending substantiallyperpendicular to the longitudinal axis of the waveguide, and themounting means includes at least one locating means for engaging theopposed surfaces of at least one of these portions so as tosubstantially prevent longitudinal movement of the parts of waveguideadjacent said at least one portion.

Thus in the temperature sensor of the present invention, the locatingmeans, by locating the outwardly projecting portion in the specifiedmanner, not only accurately positions and locates the adjacent parts ofthe waveguide longitudinally, but also substantially avoids theintroduction into the waveguide of radial stresses which could give riseto spurious reflected pulses. Furthermore, since the or each locatingmeans is arranged at a respective one of the reflecting meansconstituted by the outwardly projecting portions, any slight radialstress created thereby would merely have the effect of slightly changingthe amplitude of the reflected pulse which is already being produced bythe reflecting means.

The sensor may include a plurality of such locating means, each of whichmay have surfaces made from PTFE, silicon rubber or fluorosilicon rubberfor engaging the opposed surfaces of the projecting portions, while themounting means may further include a plurality of annular supportdevices which coaxially surround the waveguide between successivereflecting means, and each of which is adapted to laterally locate thewaveguide without gripping it tightly. Each such support devicepreferably has an internal support surface (i.e., the surface whichcontacts the waveguide) made from a resilient material, which may besilicon rubber or fluorosilicon rubber, and which may advantageouslyhave one or more layers of a fine woven metal mesh, e.g., that availableunder the trade mark KNITMESH, therein.

Alternatively and preferably, the mounting means may comprise aplurality of mounting devices each of which also serves as a respectivelocating means. In this case, each mounting device may comprise anannular device which, in use, coaxially surrounds the waveguide andwhich has a bore containing a circumferentially extending groove forreceiving a respective reflecting means, said bore further containing,on each side of said groove, at least three (and preferably four)circumferentially distributed axially extending strips of resilientmaterial, preferably of circular cross-section and preferably of siliconrubber or fluorosilicon rubber, positioned therein so as to make linecontact with, and thereby radially support, the waveguide, the ends ofthe strips adjacent said groove advantageously projecting thereinto intoabutment with said reflecting means, whereby to axially locate thewaveguide. Conveniently, each strip is mounted in a respective grooveextending axially of the bore. Thus each strip may be a push fit in,and/or bonded into, its respective groove. Each mounting device isadvantageously moulded in a thermoplastics material, preferably in twopieces which mate in a plane or planes extending radially thereof andwhich can be assembled together around the waveguide.

In a preferred embodiment of the invention, each outwardly projectingportion extends around the entire circumference of the waveguide as aflange. Thus each flange may comprise an annular member which is brazed,preferably vacuum brazed, or welded to the outside surface of thewaveguide. To facilitate this, the waveguide is conveniently circular incross-section. Additionally, the flanges preferably progressivelyincrease in size with increasing distance from said one end of thewaveguide, so as to tend to reduce the differences between therespective amplitudes of the reflected pulses arriving back at said oneend in response to a given launched pulse .

The invention also includes a temperature sensing system incorporating atemperature sensor in accordance with any of the preceding statements ofinvention, and further comprising means for launching ultrasonic pulses,for example longitudinal pulses, into one end of the waveguide, meansfor detecting reflected ultrasonic pulses due to said reflecting means,and means for measuring the respective time intervals between successiveones of said reflected pulses.

According to another aspect of the invention, there is provided amounting device for an ultrasonic waveguide, the device comprising anannular device which, in use, coaxially surrounds the waveguide andwhich has a bore containing at least three circumferentially distributedaxially extending strips of resilient material positioned therein so asto make line contact with, and thereby radially support, the waveguide.

Where the mounting device is to be used with a waveguide provided withat least one outwardly projecting portion having a pair of opposedsurfaces extending substantially perpendicular to the longitudinal axisof the waveguide, the bore of the annular device may be provided with acircumferentially extending groove for receiving said outwardlyprojecting portion, said strips may be provided separately on each sideof said circumferentially extending groove, and the end of the stripsadjacent said circumferentially extending groove may project thereintointo abutment with said outwardly projecting portion, whereby to axiallylocate the waveguide.

The invention will now be described, by way of example only, withreference to the accompanying drawings, of which:

FIG. 1 is a much simplified schematic drawing of a first embodiment of adistributed temperature sensor and system in accordance with theinvention;

FIG. 2 is a more detailed and enlarged sectional view, still somewhatschematic, of part of the sensor of FIG. 1;

FIGS. 3 and 4 are sectional views on the lines 3--3 and 4--4respectively of FIG. 2;

FIG. 5 is a partly exploded sectional view of an alternative form ofmounting fixture for use as part of the sensor of FIG. 1;

FIG. 6 is another sectional view of the mounting fixture of FIG. 5; and

FIG. 7 is a perspective view of one half of the mounting fixture of FIG.5.

The distributed temperature sensor of FIG. 1 is indicated generally at10, and comprises an elongate ultrasonic waveguide 12 in the form of acylindrical rod or wire 14 of nickel, or a nickel-based alloy such asINCONEL or NICHROME, or annealed stainless steel. The waveguide 12 istypically between 3 and 10 meters long, and is divided into a pluralityof zones, typically each about 5 to 100 cm long, as Will hereinafterbecome apparent: the lengths of the zones need not be the same, but canbe tailored to the area whose temperature is to be monitored (e.g., withseveral short zones in critical parts of the area, and fewer, longer,zones in less critical parts of the area.)

Coupled to one end 16 of the waveguide 12 is an ultrasonic pulsetransmitter and receiver 18, typically comprising a magnetostrictivedevice which launches longitudinal ultrasonic pulses (also referred toas compressional or expansional pulses) into the waveguide 12. Thetransmitter/receiver 18 is also coupled to a timing circuit 20, which isarranged to measure the time interval between each pulse launched intothe waveguide 12 and any reflected pulses resulting from that pulse.

Part of the waveguide 12 is shown in more detail in FIG. 2, where it canbe seen that the wire or rod 14 is divided into the aforementioned zonesby equally longitudinally-spaced annular flanges 22, which projectradially from the waveguide 12. Each flange 22 is constituted by anannular washer which is made of the same material as the waveguide 12,and which is either vacuum brazed to the outside of the waveguide 12using a brazing material whose melting point is higher than the highesttemperature that the sensor 10 is intended to be able to withstand, or,for fire detection applications, welded thereto, e.g., by electron beamwelding.

In use, the waveguide 12 is typically strung around an area whosetemperature is to be monitored, for example around the outside of anaircraft gas turbine engine, within the engine housing or nacelle, thewaveguide being sufficiently flexible to permit this. As mentionedhereinbefore, great care must be taken with mounting the waveguide 12 toensure that the mounting fixtures used do not introduce unwantedvariations in the acoustic impedance of the waveguide. To this end, twodifferent kinds of mounting fixture are used, as indicated at 24 and 26in FIGS. 2 to 4.

The mounting fixture 24 (FIGS. 2 and 3) comprises two generallycup-shaped annular members 28 which coaxially surround the waveguide 12and are disposed one on each side of one of the flanges 22, and one ofwhich is fixedly secured to a convenient fixed point in the area whosetemperature is to be monitored. Each of the members 28 has an axiallyextending portion 30 and a radially inwardly extending portion 32, theportions 32 defining respective apertures 34 which are greater indiameter than the flange 22.

The portions 30 have co-operating screw threads 36 at their respectiveopen ends, whereby they can be screwed together to lightly press twodiametrically split bushes 38 of PTFE into engagement with the radiallyextending surfaces of the flange 22. The bushes 38 are shaped to fitsnugly within their respective ones of the members 28, and haverespective apertures 40 which coaxially surround the main body of thewaveguide 12 with clearance. Thus as the members 28 are screwedtogether, the bushes lightly grip the flange 22 with a gripping forcedirected axially of (i.e., parallel to the longitudinal axis of) thewaveguide 12, but do not press against the main body of the waveguide12. This prevents longitudinal movement of the waveguide 12, soaccurately locating it longitudinally with respect to the area whosetemperature is to be monitored, while not applying forces thereto likelyto result in radial stresses in the main body of the waveguide 12.

In practice, the frictional engagement between the bushes 38 and theflanges 22 holds the waveguide 12 fairly securely against radial (i.e.,transverse) movement as well. However, since the mounting fixture 24 isnormally used at only a few selected ones of the flanges 22, furtherlateral support for the waveguide 12 is provided by the mountingfixtures 26.

The mounting fixture 26 comprises a pair of half-cylindrical clampingshells 42, one of which is again fixedly secured by any convenient means(not shown) to a fixed point in the area whose temperature is to bemonitored. The clamping shells 42 have a lining 44 made of fluorosiliconrubber, and clamp together so as to coaxially surround the waveguide 12and lightly compress the lining 44. Thus the mounting fixture 26provides good lateral support for the waveguide 12 without applyingforces thereto which would result in radial stresses in the main body ofthe waveguide. It does not provide good longitudinal location, but thatdoes not matter since longitudinal location is provided by the mountingfixtures 24 as already described.

FIGS. 5 to 7 show an alternative mounting fixture, generally indicatedat 50, which can replace both of the mounting fixtures 24 and 26. Themounting fixture 50 is substantially cylindrical, and is moulded in twoidentical halves 50a and 50b from a high temperature thermoplasticsmaterial, for example the material available from Morgan Matroc Ltd ofSandy, Bedfordshire, England under the name Micatherm HT. The two halves50a and 50b have diametrically extending mating surfaces 52, which areprovided with complementarily shaped spigots 54 and recesses 56 tolocate and lock them precisely with respect to each other as they aresecured together. The external cylindrical surface of the mountingfixture 50 is provided with radially outwardly projecting flanges 58 atits two ends, the recessed region 60 defined between these flangesserving to receive and locate a typical C-clamp by which the mountingfixture 50 is secured to some convenient point in the area whosetemperature is being monitored.

The mounting fixture 50 contains an axially extending central bore 62,having four equiangularly distributed circular section grooves 64extending axially along its wall. These grooves 64 intersect acircumferential groove 66, which extends completely round the bore 62midway between the two ends of the bore, and which effectively cuts eachgroove 64 into two axially separated halves. The section of each of thegrooves 64 comprises just over half the circumference of a circle, sothat the mouth 68 of each groove is very slightly narrower than themaximum width of the groove.

Each axial half of each groove 64 contains a respective circular-sectionstrip 70 of fluorosilicon rubber, which is of substantially the samediameter as its groove. Each strip 70 is therefore a snap-fit in itsgroove 64, and is bonded therein using a silicon or other hightemperature resistant adhesive. The end 72 of each strip 70 near thecircumferential groove 66 extends slightly into that groove, so as todefine with the end 72 of the strip 70 in the other half of the samegroove 64 (i.e., on the other side of the groove 66) a small gap 74.

In use, the two halves 50a and 50b of the mounting fixture 50 areassembled together around the waveguide 12 at one of the flanges 22, sothat the flange is received within the circumferential groove 66 andlocated in the small gaps 74 defined between the ends 72 of the twostrips 70 in the two halves of each groove 64, i.e., these ends 72 abutthe opposite sides of the flange. Further, the respective diameters ofthe bore 62 and of the strips 70 are carefully selected, in relation tothe diameter of the waveguide 12, to ensure that each strip 70 makeslight line contact with the waveguide, without significantly compressingthe strips 70. As a result, the strips 70 serve to radially (orlaterally) locate the waveguide 12 without applying significantcompressive forces to it, while the abutment of the ends 72 of thestrips 70 with the flange 22 within the circumferential groove 66axially locates the waveguide 12, again without applying significantloads to it. Additionally, the rubber strips 70 are loaded only incompression, even under vibration conditions, so that shear loading ofthe strips, which is extremely undesirable, is avoided.

In practice, a plurality of the mounting fixtures 50 are distributedalong the waveguide 12, say one at each third or fourth flange 22.However, some can be mounted between flanges 22 if desired, although inthat position, they will not provide the aforementioned axial locationof the waveguide.

The materials chosen for the mounting fixture 50, i.e., the hightemperature thermoplastics material and the fluorosilicon rubber, areselected to ensure that the fixture can survive a standard aircraft firetest. For less rigorous applications, other materials can be used, e.g.,silicon or other synthetic rubber for the strips 70.

To monitor the temperature, ultrasonic pulses are periodically launchedinto the end 16 of the waveguide 12 by the transmitter/receiver 18,typically at a frequency of 100 Hz. Each pulse so launched is partiallyreflected at each of the flanges 22 to form a respective echo pulse, sothat a succession of these echo pulses is received back at thetransmitter/receiver 18, one from each flange.

The propagation speed of the ultrasonic pulses in the waveguide 12 istypically about 5000 m/sec, but as already mentioned, it varies withtemperature. Thus the time interval between the receipt of any twosuccessive echo pulses at the transmitter/receiver 18 is a function ofthe average temperature of the waveguide 12 between the two successiveflanges 22 which produced those two echo pulses, and therefore anindication of the average temperature of the zone of the area whosetemperature is being monitored defined by those two flanges. Therespective time intervals between each pair of successive echo pulses istherefore measured by the timing circuit 20.

In addition to the echo pulses produced by the flanges 22, a later andsignificantly larger echo pulse is reflected from the far end 46 of thewaveguide 12 (i.e., the end remote from the end 16). The detection ofthis larger end-reflected pulse serves as a useful check that thewaveguide is unbroken and transmitting pulses satisfactorily. Moreimportantly, the time of its arrival gives an indication of the averagetemperature of the waveguide 12 as a whole, which is used by amicroprocessor within the transmitter/receiver 18 to adjust the temporalposition, with respect to each launched pulse, of a plurality ofsuccessive time "gates" or "windows" within which each of the successiveecho pulses from the flanges 22 ought to occur. This can enable spuriouspulses due to external interference to be discriminated from true echopulses, and so improve the reliability of the system.

The amplitude of each echo pulse received back at thetransmitter/receiver 18 from a given flange 22 is a function of theamplitude of the ultrasonic pulse arriving at that flange, the physicalsize (diameter, thickness) of the flange, and to a lesser extent, thedistance of the flange from the transmitter/receiver. So since eachflange 22 typically reflects about 1% to 4% of the ultrasonic energyarriving at it, each successive flange in the direction of propagationof the pulses launched by the transmitter/receiver 18 has less energy toreflect than the preceding one, so that successive echo pulses getprogressively smaller in amplitude. However, in order to simplify thedetection and processing of the echo pulses in the transmitter/receiver18, it is desirable that they be of approximately similar amplitude. Soto achieve this, each successive flange 22 in the direction ofpropagation of the pulses launched by the transmitter/receiver 18 isslightly larger than the preceding one, as shown in greatly exaggeratedform in FIG. 2: the calculation of the relative sizes of the flanges 22to produce this result is a relatively simple matter. A similar resultcan be achieved, or the result can be assisted, by including a variablegain amplifier whose gain is increased slightly for each successive echopulse within the input of the receiver section of thetransmitter/receiver 18.

Many modifications can be made to the described embodiment of theinvention. In particular, the lining 44 in the mounting fixture 26 canhave one or more layers of fine woven metal mesh, such as that availableunder the trade mark KNITMESH, bonded therein, so as to maintain theintegrity of their function for a certain minimum period of time evenwhen the rubber is degraded by high temperature, e.g., due to fire.

Also to facilitate assembly and maintenance, the waveguide 12 can beconnected to the transmitter/receiver 18 by means of an ultrasonicwaveguide connector of the kind described in our co-pending UnitedKingdom Patent Application No. 8814246, entitled "Ultrasonic WaveguideConnector" and filed on June 15, 1988.

Additionally, suitable materials other than those specifically cited canbe used to make the waveguide 12, and shapes other than cylindrical,e.g., rectangular-section or flat ribbon shapes, can also be used.Further, for some shapes of waveguide, ultrasonic pulses other thanlongitudinal pulses can be used, for example torsional pulses, and thepulses can in some cases be produced by a piezoelectric device ratherthan a magnetostrictive device.

We claim:
 1. A temperature sensor comprising an elongate ultrasonicwaveguide having distributed along its length a plurality of means forpartially reflecting ultrasonic pulses launched into one end of thewaveguide by an ultrasonic pulse transmitting and receiving meanscoupled to said one end of said waveguide, and means for mounting thewaveguide such that it extends through an area whose temperature is tobe monitored, wherein at least some of the reflecting means compriseoutwardly projecting portions of the waveguide each having a pair ofopposed surfaces extending substantially perpendicular to thelongitudinal axis of the waveguide, and the mounting means includes atleast one locating means for engaging the opposed surfaces of at leastone of these portions so as to substantially prevent longitudinalmovement of the parts of the waveguide adjacent said at least oneportion.
 2. A temperature sensor as claimed in claim 1, wherein thesurfaces of the locating means which engage the opposed surfaces of theprojecting portion or portions are made from a material selected fromthe group of materials comprising PTFE, silicon rubber and fluorosiliconrubber.
 3. A temperature sensor as claimed in claim 1, wherein themounting means further includes at least one annular support devicewhich coaxially surrounds the waveguide, and which is adapted tolaterally locate the waveguide without gripping it tightly.
 4. Atemperature sensor as claimed in claim 3, wherein the support device hasan internal support surface made from a resilient material.
 5. Atemperature sensor as claimed in claim 4, wherein said material isselected from the group of materials comprising silicon rubber andfluorosilicon rubber.
 6. A temperature sensor as claimed in claim 1,wherein the mounting means comprises a plurality of mounting devices,each of which also serves as a respective locating means.
 7. Atemperature sensor as claimed in claim 6, wherein each mounting devicecomprises an annular device which, in use, coaxially surrounds thewaveguide and which has a bore containing a circumferentially extendinggroove for receiving a respective reflecting means, said bore furthercontaining, on each side of said groove, at least threecircumferentially distributed axially extending strips of resilientmaterial positioned therein so as to make line contact with, and therebyradially support, the waveguide.
 8. A temperature sensor as claimed inclaim 7, wherein the ends of the strips adjacent said groove projectthereinto into abutment with said reflecting means, whereby to axiallylocate the waveguide.
 9. A temperature sensor as claimed in claim 7,wherein there are four such strips, equiangularly distributed around thebore.
 10. A temperature sensor as claimed in claim 7, wherein each stripis of circular cross-section.
 11. A temperature sensor as claimed inclaim 7, wherein each strip is mounted in a respective groove extendingaxially of the bore, and is either a push fit in, or bonded into, itsgroove.
 12. A temperature sensor as claimed in claim 7, wherein eachannular device is made in two pieces which mate in a plane or planesextending radially thereof and which can be assembled together aroundthe waveguide.
 13. A temperature sensor as claimed in claim 1, whereineach outwardly projecting portion extends around the entirecircumference of the waveguide as a flange.
 14. A temperature sensor asclaimed in claim 13, wherein each flange comprises an annular memberwhich is brazed to the outside surface of the waveguide.
 15. Atemperature sensor as claimed in claim 13, wherein the flangesprogressively increase in size with increasing distance from said oneend of the waveguide, so as to tend to reduce the differences betweenthe respective amplitudes of the reflected pulses arriving back at saidone end in response to a given launched pulse.
 16. A temperature sensoras claimed in claim 1, wherein said ultrasonic pulse transmitting andreceiving means comprises means for launching ultrasonic pulses into oneend of the waveguide and means for detecting reflected ultrasonic pulsesdue to said reflecting means, and said temperature sensor furthercomprising means for measuring the respective time intervals betweensuccessive ones of said reflected pulses.